In diagnostic X-ray equipment unwanted X-ray scatter can be reduced by interposing a rotating X-ray-opaque masking disk with X-ray transparent slits between an X-ray tube and the subject under examination. These slits are sectoral with their divergent long sides defined by radii from the axis of the disk, and serve to confine the X-rays to a fan shaped beam with a narrow sectoral cross section sweeping across the subject. Examples of slit radiography appear in U.S. Pat. Nos. 2,730,566 (Bartow), 4,315,146 (Rudin), and 4,404,591, (Bonar).
A disk using a single slit for each exposure sweep must be sector shaped for uniform exposure across the image area of the subject, but gives rise to non-uniform scatter of X-rays from the subject. Scatter is quantified by the ratio S/P of X-rays scattered from the subject to the primary rays forming the true image of the subject. This scatter-to-primary ratio S/P is not uniform over the image area scanned by a single sector-shaped beam, and may be non-uniform when disks with multiple slits are used to form a plurality of fan-shaped beams for each scan in a single X-ray exposure.
I have discovered that uniform exposure and uniform scatter-to-primary ratio S/P over an image area can be achieved with multiple sector slits provided certain limits of the geometric parameters of the system are observed. These limits are expressed in terms of dimensionless ratios and are thus general in scope.